Neuroscientist David Linden thinks that of the five senses, touch is the most overlooked, and perhaps the most important for promoting psychological health.

About David Linden

Dr. David J. Linden is a professor in the Solomon H. Snyder Department of Neuroscience at the Johns Hopkins University School of Medicine. He served for many years as the Chief Editor of the Journal of Neurophysiology. He has written several books, including Touch: The Science of Hand, Heart and Mind.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.

GUY RAZ, HOST:

Today on the show, ideas about the five senses. And we've now made it through sound, taste, sight and smell, which leaves touch.

DAVID LINDEN: There are things we don't understand about all the senses. But touch has been particularly underexplored. If you look in the scientific literature, you'll find, probably, a hundred papers on vision for every one about touch.

RAZ: This is David Linden.

LINDEN: I'm a professor of neuroscience at Johns Hopkins University School of Medicine.

RAZ: And so, as a neuroscience professor, David researches how our brain processes all of the five senses. But he says touch gets the least attention.

LINDEN: Part of it is a failure of imagination. Sighted people can close their eyes and kind of imagine what it would be like to be blind. Or we can plug our ears...

RAZ: Yeah.

LINDEN: ...And we can imagine what it's like to be deaf.

RAZ: Right.

LINDEN: But there's no way to imagine losing your sense of touch. And I think that makes it somehow less compelling.

RAZ: But David says it shouldn't be. Here he is on the TED stage.

(SOUNDBITE OF TED TALK)

LINDEN: We know that if you're born blind - without sight - that you can develop a great mind and a great body and have a great life. And, likewise, if you're born deaf, you can develop a great mind and a great body and have a great life. But if you're born with the biological components for a sense of touch, but you do not receive touch during your infancy and when you're a toddler, then a disaster unfolds.

And we know this mostly from what happened in orphanages in Romania during the Ceausescu regime in the 1970s and what followed in the 1980s. And these were situations where there were grossly understaffed orphanages. There was no one to hug or hold or be loving in a tactile way towards these children. And they developed horrible, compulsive, self-soothing rocking motions. Later, they had attachment disorders, cognitive delays.

And it wasn't just neuropsychiatric problems. Their growth was stunted. And they had problems with the development of their gastrointestinal systems and their immune systems. And we know that this is because of touch deprivation because, in some cases, volunteers came in and gave just 30 minutes a day of loving touch - a little hugging, a little limb manipulation. And that was enough to completely reverse all of these deleterious effects but only if it occurred in the first two years of life. If the intervention came after age 2, all those problems would persist for the rest of life.

RAZ: Why? I mean, what explains it? I mean, it seems strange that this external experience - that someone touching you or you touching something else would have internal implications.

LINDEN: It is strange. And the truth - the embarrassment for biologists - is that we don't understand it. In other words, I wish I could give you a pathway of cells and molecules that went from loving touch to proper development of the gastrointestinal or immune system. But we don't understand that.

RAZ: Is it - is touch real, or is it an expression of a signal that your brain is sending to you?

LINDEN: Well, all sensation, whether it's vision or touch or hearing or what have you, only occurs in the brain. So if you interrupt the pathway from the body to the brain, as can occur, for example, in spinal-cord injury, then those sensations won't occur. That doesn't make it, to my mind, any less real. But it does point out the fact that our sensations, touch included, are actively constructed in our brains.

And our brains work not to give us the most accurate representation of the external world - not the pure, unalloyed lowdown. But they are spinning the data. They are ignoring some information, emphasizing others and mixing the stew together in a way that the brain thinks, through evolution, will be most useful.

(SOUNDBITE OF TED TALK)

LINDEN: Imagine that you're walking down the street. And as you walk down the street, you're moving your limbs and your torso. And your clothes are moving against your body. And you're not thinking about it at all. It doesn't enter your consciousness one bit. Those sensations are strongly suppressed, whereas, if you imagine you're stopped on the street corner, and now those same sensations come on your body, oh, you'd be very attentive to them. They would have great salience. It's because we're hardwired to suppress the sensations that result from our own motion. And this makes evolutionary sense - right? - because the outside world - that's where the things are that we might want to eat, that we might want to mate with, that we might want to run away from. So we want to pay more attention to the outside world than to the consequences of our own motions. And the crucial medical issue that is tied up with this is why it is that it's so very hard to tickle yourself, right?

(LAUGHTER)

LINDEN: So when you go to tickle yourself, electrical signals are flowing from the motor cortex in your brain down to the muscles of your arm and your hand to produce that tickling motion. But a copy of those signals is going to a part of the brain called the cerebellum. And the cerebellum transforms those into inhibitory signals and suppresses those sensations.

RAZ: You know, I wonder. I mean, aside from the biology, I mean, there's a social aspect to touch, right? I mean, does touch affect how we think about other people, like, how we interact with other people?

LINDEN: Absolutely. We are specialized by many years of evolutionary history to extract social information from interpersonal touch. It is incredibly dependent upon social context. And it's incredibly dependent on cultural contexts. This was really revealed by Sidney Jourard, a psychologist who liked to spy on people in cafes in the 1960s. And he would go all around the world. And he would find that in San Juan, Puerto Rico, people would touch each other on the order of 200 times in an hour.

These weren't as lovers. These would be friends or colleagues from work. And then he would do the same thing in Paris - 40 times an hour. And in New York City - two times an hour. In London - zero times an hour. And this just points out that there's a lot going on that isn't just hardwired into our biology. Our social, cultural expectations about touch really influence how we feel about it.

RAZ: That's amazing. I mean, it hews to kind of a cultural stereotype that we have that people in Latin America are warmer and more generous and friendlier. And the more sort of north you get into Europe, people are colder and less willing to engage right away.

LINDEN: That's right. And there are other places where there is a lot of social touching allowed within the sexes. But between the sexes, it's completely banned outside of small children or marriage. I think the crucial thing to keep in mind is that the way we experience touch is utterly dependent upon context. Imagine you're having an argument with your sweetheart. And then your sweetheart reaches out and strokes your arm in the middle of the argument. It's not resolved. How does that feel? It doesn't feel good at all. It feels unwanted.

Now, imagine the very same touch on your arm - the very same pressure velocity from the very same person but now in a loving connect time. It feels completely different. And it's not that the physical sensation's the same, and we just think about it differently afterwards. From the very first moment you're aware of that sensation, it feels differently. And that's because our experience of touch is conditioned by context.

RAZ: Yeah. I want to ask about the vocabulary around this sense because it's fundamental to the way we interact with each other, right? We say, I'm touched by your gesture. Or my feelings get hurt. You know, these words that we use are so connected to this sense. What explains it?

LINDEN: Well, I think the important thing to realize is that this isn't just a quirk of modern-day English. You can go back in English, or you can look at many other languages around the world - not just from the Indo-European language group. And this construction is present not in every language but very broadly around the world. So I think that is telling us something that is fundamental to being human. And that is there is a deep link between the sense of touch and emotion. And that link is hardwired in our brains.

RAZ: David Linden - he's a professor at Johns Hopkins University in Baltimore. You can see his entire talk at ted.com.

(SOUNDBITE OF SONG, "SENSES WORKING OVERTIME")

XTC: (Singing) And all the world is football-shaped. It's just for me to kick in space. And I can see, hear, smell, touch, taste. And I've got one, two, three, four, five senses working overtime.

RAZ: Hey, thanks for listening to our show on the five senses this week. If you want to find out more about who was on it, go to ted.npr.org to see hundreds more TED Talks. Check out ted.com or the TED app. Our production staff at NPR includes Jeff Rogers, Brent Bachman, Megan Kane (ph), Neva Grant and Sanaz Meshkinpour, with help this week from Casey Herman, Chris Benderev, Rachel Faulkner, Camilo Garzon and Daniel Shukin. Our intern is Thomas Lu.

Our partners at TED are Chris Anderson, Kelly Stoetzel, Anna Phelan and Janet Lee. If you want to let us know what you think about the show, you can write us at tedradiohour@npr.org. You can follow us on Twitter. It's @TEDRadioHour. I'm Guy Raz. And you've been listening to ideas worth spreading right here on the TED Radio Hour from NPR.